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Motor Speed Control Module - Part 2

A module that automatic varies intercooler pump or fan speed with engine load.

by Julian Edgar

Click on pics to view larger images

By building this module you can automatically vary the speed of an intercooler fan (or the water pump in a water/air intercooler system) so that as engine load increases, the pump or fan speed automatically goes up as well. It's cheap, effective and if you've built an electronic kit before, easy to build.

  • Cheap DIY kit construction
  • Variable speed electric motor control module
  • Speed of the pump or fan increases with engine load
  • 'Idle up' control to boost slowest setting of pump or fan speed

Ideal for

  • Water/air intercooler pump control
  • Speed control for some air/air intercooler fans

Last week we covered how to build the Motor Speed Control - if you are reading this article without having first read that one, go read Part 1!

As built, the module allows you to control the speed of a 12V load, and it is especially designed to control a 12V motor. By turning the trimpot on the module, the output voltage is altered and so the motor speed changes as well. As discussed in the last article, the module is capable of controlling some pretty hefty loads - 10 amps if only one mosfet is fitted, and 20 amps if you buy the optional extra. (Most fans and pumps take less than 10 amps - and even a monster will very seldom draw 20 amps!)

Click for larger image

However, if you're using the module to control the speed of a water/air intercooler circulation pump (or an air/air intercooler fan), you don't want to be turning a knob every time that you'd like its speed to change. Instead, wouldn't it be good if as the engine load increases, the pump or fan speed automatically went up as well? That way, there's better intercooling when it's required.

In addition to having the module respond to engine load, it would also be really good if you could set the minimum speed that the pump will drop to off-load. After all, it's not a great help if the pump completely stops working every time you're driving gently...

So there are the two requirements:

  • Automatic speed variation with engine load
  • Adjustable minimum speed

And the good news is that both can be very easily achieved.

The Load Signal

Click for larger image

To be able to use the module in this application you'll need just one extra component - a 100 kilo-ohm pot. This pot allows you to adjust the slowest speed that the pump or fan will rotate at. However, before you can use it, you need to work out where the load signal on your car is going to come from.

The load signal can be derived from either the MAP sensor or airflow meter, depending on which one your engine has. You can tap into its load output without worrying the car's ECU - the airflow meter or MAP sensor can simultaneously work with both the factory ECU and also this added module. (The module draws about 2.5 milliamps from the airflow meter or MAP sensor circuit.)

So how do you actually connect it? Let's start off with an airflow meter.

Click for larger image

It's best if you have the workshop manual, but you should still be able to find the right wire even if you don't. You'll need your good quality digital multimeter, set to measure voltages. Earth one side of the meter then use the other (sharp) probe to carefully backprobe the airflow meter connector. Typically, you'll find 0 volts (meter earth), 5 volts (regulated power supply to the meter), and a voltage that varies as you rev the engine up and down. There will also be other connections for temperature outputs, so keep on probing until you find the right wire.

Click for larger image

Remove a short section of insulation from the signal output wire and solder a new wire to it, either at the meter itself or if you want to follow the colour codes (and have that workshop manual!), back at the ECU itself. Run the new wire into the cabin and connect the multimeter to it and earth, so that the voltage output on this wire can be read off as the car is being driven. Use an assistant to read the meter.

Confirm that the signal voltage does rise with increased load, and that it peaks at around 4 - 5 volts.

Working with a MAP sensor is a similar process, except it's likely that the MAP sensor will only have three connections - power, earth and the signal output. But again backprobe the connector until you find the wire that has a voltage that varies with engine load.

Note that some airflow meters and MAP sensors have frequency outputs, rather than varying voltages. This module will not work with frequency output sensors.

Modifying the Module

As covered last week, the module is a DIY kit. It needs to build according to the instructions and then checked that it is working correctly. Then, once you have done that, the design can be modified slightly for its new application.

Click for larger image

The first step is to remove the 5K trimpot that is on the printed circuit board. If you use a solder sucker, it's fairly easy to unsolder this component. With it off and the board orientated as in this photo, you'll see a triangular array of blank spots where the legs of the trimpot were previously soldered. Two of these vacant connections will be used with the new pot, while the third is left unused.

Here we've used a full-size 100 kilo-ohm pot, which can easily be remote mounted.

The centre (wiper) terminal of the new pot goes to the solder pad closest to the edge of the board, where the old pot's wiper arm also went. One side of the pot goes to the lower solder pad, as shown here. The other side of the new pot is connected to the output signal from the airflow meter or MAP sensor (the pink wire here).

The new pot allows you to increase the speed that the pump or fan turns at when the signal coming out of the load sensor is low (eg when the car is idling). If the fan or pump turns too slowly in these conditions, you can use the pot to increase its speed.

Other Uses?

There are lots of ways of controlling the module that we haven't gone into. Basically, all you need is a 0-5 volt input signal. So, if you make some measurements and find that the throttle position sensor outputs (say) 1-4 volts and increases its output with throttle opening, then you could easily use that signal to trigger the module instead.


In this example of how to test the system I have used a water pump, but instead you can use an electric fan - or even just a 12-volt warning light, whose brightness will vary with the different voltages coming out of the module.

  1. Connect the pump to the module's outputs, connect earth and 12V, and connect the airflow meter or MAP sensor signal to the module through the new pot, all as described above.
  2. Start the car and turn the pot until the pump rotates at its slowest speed
  3. Assess whether this is too slow during engine idle conditions. If it is, increase its speed by turning the knob.
  4. Check that at high loads the pump progressively goes to full power.

Easy, huh?

You should really only need to set the pump/fan 'idle speed control' once, so when the system is set up, the box containing the module can be buried somewhere inside the car's cabin. (But here's a thought - if you wanted, you could mount the pot on the dash so that you have an easy override control. When you're racing, you could then easily run the fan or pump flat-out all of the time, just by turning the 'idle control' knob right up.)

If you want a simple monitor of the pump or fan speed, just wire a 12V filament lamp warning light in parallel with the fan or pump, and mount the lamp on the dash. That way, as more voltage gets fed to the fan or pump, the lamp gets brighter.

Note that there is always going to be a small voltage drop through the module - even at full power. Typically, this is about 1.4V - so for critical applications like fuel pump control in a very high powered-car, that may be too high a voltage drop.

Available in the AutoSpeed Shop : Kit KC5225 '10A 12VDC Motor Speed Controller Kit'

Next week: modifying the module to run a water injection system!

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